Methods for Forming Recesses in Source/Drain Regions and Devices Formed Thereof

ABSTRACT

Embodiments disclosed herein relate generally to methods for forming recesses in epitaxial source/drain regions for forming conductive features. In some embodiments, the recesses are formed in a two-step etching process including an anisotropic etch to form a vertical opening and an isotropic etch to expand an end portion of the vertical opening laterally and vertically. The recesses can have increased contact area between the source/drain region and the conductive feature, and can enable reduced resistance therebetween.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. application Ser. No. 15/992,598, filed on May 30, 2018, entitled “Methods for Forming Recesses in Source/Drain Regions and Devices Formed Thereof” which is hereby incorporated herein by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. When a semiconductor device such as a fin field-effect transistor (FinFET) is scaled down through various technology nodes, several strategies have been employed to improve device performance, such as using high-k dielectric materials and metal gate electrode structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1 and 2A-2B, 3A-3B, 4A-4B, 5A-5B, 6A-6B, and 7A-7B are schematic three-dimensional and cross-sectional views of a portion of a semiconductor device corresponding to various stages in an example manufacturing process according to some embodiments.

FIGS. 8A-8B and 9A-9B are schematic cross-sectional views of a portion of a semiconductor device corresponding to various stages in another example manufacturing process according to some embodiments.

FIG. 10 illustrates a portion of the cross-sectional view of FIG. 5A to further illustrate additional details in accordance with some embodiments.

FIG. 11 illustrates a portion of the cross-sectional view of FIG. 6A to further illustrate additional details in accordance with some embodiments.

FIG. 12 illustrates a portion of the cross-sectional view of FIG. 7A to further illustrate additional details in accordance with some embodiments.

FIG. 13 illustrates a portion of the cross-sectional view of FIG. 8A to further illustrate additional details in accordance with some embodiments.

FIG. 14 illustrates a portion of the cross-sectional view of FIG. 9A to further illustrate additional details in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Embodiments disclosed herein relate generally to methods for forming recesses in epitaxial source/drain regions for forming conductive features. In some embodiments, the recesses are formed in a two-step etching process including an anisotropic etch and an isotropic etch. The recesses can have increased contact area between the source/drain region and the conductive feature, and can enable reduced resistance therebetween.

The foregoing broadly outlines some aspects of embodiments described in this disclosure. It is contemplated that the concepts of the present disclosure may be implemented for a planar transistor device or for a three-dimensional transistor device, such as the semiconductor device 240 described in this disclosure. Some example devices for which aspects described herein may be implemented include fin field effect transistors (FinFETs), Horizontal Gate All Around (HGAA) FETs, Vertical Gate All Around (VGAA) FETs, nanowire channel FETs, strained-semiconductor devices, silicon-on-insulator (SOI) devices, or other devices.

FIG. 1 illustrates an example of the semiconductor device 240 in a three-dimensional view. FIGS. 2A-2B through 7A-7B and 8A-8B through 9A-9B are schematic cross-sectional views of a portion of the semiconductor device 240 corresponding to various stages of fabrication according to some embodiments. It is noted that the methods described herein may be utilized to form any other semiconductor structures not presented herein. Those having ordinary skill in the art should recognize that the full process for forming a semiconductor device and the associated structures are not illustrated in the drawings or described herein. Although various operations are illustrated in the drawings and described herein, no limitation regarding the order of such steps or the presence or absence of intervening steps is implied. Operations depicted or described as sequential are, unless explicitly specified, merely done so for purposes of explanation without precluding the possibility that the respective steps are actually performed in concurrent or overlapping manner, at least partially if not entirely.

As shown in FIG. 1, the semiconductor device 240 has a fin 274 formed on a semiconductor substrate 270. The semiconductor substrate 270 may be or include a bulk semiconductor substrate, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. In some embodiments, the semiconductor material of the semiconductor substrate 270 may include an elemental semiconductor including silicon (Si) or germanium (Ge); a compound semiconductor; an alloy semiconductor; or a combination thereof.

Each fin 274 provides an active area where one or more devices are formed. The fin 274 is fabricated using suitable processes including masking, photolithography, and/or etch processes to form trenches 253 in the substrate 270, leaving the fin 274 extended upwardly from the substrate 270. The trenches 253 may then be filled with an insulating material such as an oxide (e.g., silicon oxide), a nitride, the like, or a combination thereof. The insulating material may be recessed, such as by using an acceptable etch process, to form the isolation regions 278. The insulating material is recessed such that the fin 274 protrudes above and from between neighboring isolation regions 278.

The semiconductor device 240 includes gate structures 251 formed over top surfaces and along sidewalls of the fin 274. The gate structures 251 extend longitudinally perpendicularly to the fin 274. Each gate structure 251 includes a gate dielectric 280, a gate layer 282 over the gate dielectric 280, and a mask 284 over the gate layers 282. The semiconductor device 240 also includes source/drain regions 292 disposed in opposing regions of the fin 274 with respect to the gate structures 251.

The gate structures 251 can be operational gate stacks in a gate-first process or can be dummy gate stacks in a replacement gate process. For simplicity, a replacement gate process is described herein; a person having ordinary skill in the art will readily understand modifications to processing described herein to implement a gate-first process. In a replacement gate process, the gate dielectric 280 may be an interfacial dielectric, and the gate layer 282 may be a dummy gate. The gate dielectrics 280, the gate layers 282, and the mask 284 for the gate structures 251 may be formed by sequentially forming respective layers, such as by appropriate deposition techniques, and then patterning those layers into the gate structures 251, such as by appropriate photolithography and etching processes. The interfacial dielectrics may include or be silicon oxide, silicon nitride, the like, or multilayers thereof. The dummy gates may include or be silicon (e.g., polysilicon) or another material. The masks may include or be silicon nitride, silicon oxynitride, silicon carbon nitride, the like, or a combination thereof.

FIG. 1 further illustrates reference cross-sections that are used in later figures. Cross-section A-A is in a plane along, e.g., channels in the fin 274 between opposing source/drain regions 292. Cross-section B-B is in a plane perpendicular to cross-section A-A and is across a source/drain region 292 in the fin 274. Subsequent figures refer to these reference cross-sections for clarity. The following figures ending with an “A” designation illustrate cross-sectional views at various instances of processing corresponding to cross-section A-A, and the following figures ending with a “B” designation illustrate cross-section views at various instances of processing corresponding to cross-section B-B. FIGS. 2A and 2B illustrate in the respective cross-sectional views the semiconductor device 240 depicted in the three-dimensional view of FIG. 1.

In FIGS. 3A and 3B, gate spacers 286 are formed along sidewalls of the gate structures 251 (e.g., sidewalls of the gate dielectrics 280, gate layers 282, and masks 284) and over the fin 274. The gate spacers 286 may be formed by conformally depositing one or more layers for the gate spacers 286 and anisotropically etching the one or more layers, for example. The one or more layers for the gate spacers 286 may include a material different from the material(s) for the gate structure 251. In some embodiments, the gate spacers 286 may include or be a dielectric material, such as silicon oxygen carbide, silicon nitride, silicon oxynitride, silicon carbon nitride, the like, multi-layers thereof.

After the gate spacers 286 are formed, source/drain regions 292 may be formed in the fin 274, as depicted in FIGS. 3A and 3B. In some examples, such as illustrated in the figures, recesses can be etched in the fin 274 using the gate structures 251 as masks (such that recesses are formed on opposing sides of the gate structures 251), and a material may be epitaxially grown in the recesses to form the source/drain regions 292. Additionally or alternatively, the source/drain regions 292 may be formed by implanting dopants into the fins 274 and/or the epitaxial source/drain regions 292 using the gate structures 251 as masks (such that the source/drain regions are formed on opposing sides of the gate structures 251).

Depending on the conductivity type of the transistor, the material for the source/drain regions 292 may be chosen to include or be silicon germanium, silicon carbide, silicon phosphorus, silicon carbon phosphorus, germanium, a III-V compound semiconductor, a II-VI compound semiconductor, or the like. In some examples, SiGe may be included in the source/drain regions 292 for p-type devices, while SiCP or SiP may be included in the source/drain regions 292 for n-type devices. As illustrated in FIG. 3B, due to blocking by the isolation regions 278, the material in the source/drain regions 292 is first grown vertically in recesses, during which time the source/drain regions 292 do not grow horizontally. After the recesses are fully filled, the material for the source/drain regions 292 may grow both vertically and horizontally to form facets, which may correspond to crystalline planes of the semiconductor substrate 270. In some examples, different materials are used for epitaxy source/drain regions for p-type devices and n-type devices. Appropriate masking during the recessing or epitaxial growth may permit different materials to be used in different devices.

As shown in FIG. 3A, the epitaxial source/drain regions 292 are formed between the gate structures 251 in place of the removed portions of the fin 274. In some embodiments, a cross-sectional area of the epitaxial source/drain regions 292 along the A-A line may be shaped like a hexagon, although other cross-sectional areas may be implemented, such as other shapes due to the etching process used to recess the fin 274. The cross-sectional area along the A-A line may have a height 292 h and a width 292 w. In some embodiments, the height 292 h may be in a range from about 40 nm to about 50 nm. In some embodiments, the width 292 w may be greater than 30 nm, for example, in a range from about 30 nm to about 50 nm.

In FIG. 3B, the epitaxial source/drain regions 292 are grown epitaxially from the fin 274 along a recess between the isolation regions 278. The epitaxial source/drain region 292 extends upward from the recess between the isolation regions 278 forming a substantially rhombus shape along the B-B line due to the crystallographic orientation of the material being grown, although other shapes may be formed. In some embodiments, the epitaxial source/drain regions 292 may have a rhombus cross-sectional area having a top angle 292Q along the B-B line. In some embodiments, the top angle 292Q is in a range from about 60 degrees to about 160 degrees, for example from about 70 degrees to about 80 degrees.

In the example shown in FIGS. 3A and 3B, the epitaxial source/drain regions 292 have varying widths along the respective cross-sections as the epitaxial source/drain regions 292 are traversed from respective bottom portions to top portions of the epitaxial source/drain regions 292. The width increases from the top portion of the respective epitaxial source/drain region 292 to an intermediate portion of the epitaxial source/drain region 292, and then decreases from the intermediate portion of the epitaxial source/drain region 292 to the bottom portion of the epitaxial source/drain region 292.

In FIGS. 4A and 4B, after formation of the source/drain region 292, an contact etch stop layer (CESL) 296 is conformally formed on surfaces of the source/drain regions 292, sidewalls and top surfaces of the gate spacers 286, top surfaces of the masks 284, and top surfaces of the isolation regions 278. A first interlayer dielectric (ILD) 297 is formed on the CESL 296. The CESL 296 and first ILD 297 can be deposited using any suitable deposition technique. The CESL 296 may include or be silicon nitride, silicon carbon nitride, silicon carbon oxide, carbon nitride, the like, or a combination thereof. The first ILD 297 may include or be silicon dioxide, a low-k dielectric material (e.g., a material having a dielectric constant lower than silicon dioxide), silicon oxynitride, phosphosilicate glass (PSG), borosilicate glass (BSG), borophosphosilicate glass (BPSG), undoped silicate glass (USG), fluorosilicate glass (FSG), organosilicate glass (OSG), SiO_(x)C_(y), Spin-On-Glass, Spin-On-Polymers, silicon carbon material, a compound thereof. A chemical mechanical polishing (CMP) process may then be performed to planarize the first ILD 297 and the CESL 296 and to remove the masks 284 of the gate structures 251, thereby leveling the top surface of the first ILD 297 and CESL 296 with the top surfaces of the gate layers 282.

After the CMP process, the gate structures 251 are removed using one or more etch processes so that replacement gate structures 228 a, 228 b can be formed in the recesses formed by removing the gate structures 251. Upon removal of the gate structures 251, recesses are formed between the gate spacers 286 where the gate structures 251 are removed, and channel regions of the fins 274 are exposed through the recesses. The replacement gate structures 228 a, 228 b are then formed in the recesses where the gate structures 251 were removed.

The replacement gate structures 228 a, 228 b each may include, as illustrated in FIG. 4A, an interfacial dielectric 220, a gate dielectric layer 222, one or more optional conformal layers 224, and a gate metal fill 226. The interfacial dielectric 220 is formed on a top surface and sidewalls of the fin 274 along the channel regions. The interfacial dielectric 220 can be an oxide (e.g., silicon oxide), nitride (e.g., silicon nitride), and/or another dielectric layer using any suitable deposition technique.

The gate dielectric layer 222 can be conformally deposited in the recesses where gate stacks were removed (e.g., on the interfacial dielectric 220 and sidewalls of the gate spacers 286) and on the top surfaces of the first ILD 297, the CESL 296, and gate spacers 286. The gate dielectric layer 222 can be or include silicon oxide, silicon nitride, a high-k dielectric material, multilayers thereof, or other dielectric material. A high-k dielectric material may have a k value greater than about 7.0, and may include a metal oxide of or a metal silicate of hafnium (Hf), aluminum (Al), zirconium (Zr), lanthanum (La), magnesium (Mg), barium (Ba), titanium (Ti), lead (Pb), multilayers thereof, or a combination thereof.

The one or more optional conformal layers 224 can include one or more barrier and/or capping layers and one or more work-function tuning layers. The one or more barrier and/or capping layers can include tantalum nitride, titanium nitride, the like, or a combination thereof. The one or more work-function tuning layer may include or be aluminum titanium carbide, aluminum titanium oxide, aluminum titanium nitride, the like, or a combination thereof. The materials for the one or more work-function tuning layer, barrier layer, and/or capping layer are selected so that a desired threshold voltage (Vt) is achieved for the transistor, which could be a p-type transistor or an n-type transistor. The gate metal fill 226 is formed over the one or more conformal layers 224, if implemented, and/or the gate dielectric layer 222. The gate metal fill 226 can fill remaining recesses where the gate structures 251 were removed. The gate metal fill 226 may be or include a metal-containing material such as tungsten, cobalt, aluminum, ruthenium, copper, multi-layers thereof, a combination thereof, or the like.

A planarization process, like a CMP, may remove portions of the layer for the gate metal fill 226, one or more conformal layers 224, and gate dielectric layer 222 above the top surfaces of the first ILD 297, the CESL 296, and gate spacers 286. The replacement gate structures 228 including the gate metal fill 226, one or more conformal layers 224, gate dielectric layer 222, and interfacial dielectric 220 may therefore be formed.

In FIGS. 5A and 5B, after the planarization process, a second ILD 230 is formed over the replacement gate structures 228, first ILD 297, gate spacers 286, and CESL 296. The second ILD 230 may include or be silicon dioxide, a low-k dielectric material, silicon oxynitride, PSG, BSG, BPSG, USG, FSG, OSG, SiO_(x)C_(y), Spin-On-Glass, Spin-On-Polymers, silicon carbon material, a compound thereof, a composite thereof, the like, or a combination thereof, and may be deposited using any acceptable deposition technique.

After the second ILD 230 is formed, source/drain contact openings are formed through the second ILD 230, the first ILD 297, and the CESL 296 to the source/drain regions 292 to expose at least portions of the source/drain regions 292. According to some embodiments, the source/drain contact openings may be formed through a two-step etching process, which includes an anisotropic etching process to form vertical openings 232 as shown in FIGS. 5A and 5B and an isotropic etching process to increase surface area at the bottom of the vertical opening as shown in FIGS. 6A and 6B and FIGS. 8A and 8B.

In FIGS. 5A and 5B, vertical openings 232 are formed through the second ILD 230, the first ILD 297, and the CESL 296 using photolithography and one or more anisotropic etch processes. The one or more etch processes may be a dry etching process, a Deep Reactive-Ion Etching (DRIE) process, or any suitable anisotropic etch process.

FIG. 10 is an enlarged view of region 300 in FIG. 5A showing details of a bottom portion of the vertical opening 232. The vertical openings 232 may be holes or trenches extending into the epitaxial source/drain regions 292. The sidewalls of the source/drain vertical openings 232 are substantially vertical, although there may be a small tilt angle. The vertical opening 232 has a width 232 w near a bottom surface 232 b of the vertical opening 232. In some embodiments, the width 232 w may be in a range from about 10 nm to about 40 nm. In some embodiments, the width 232 w may be in a range from about 33% to about 100% of the width 292 w of the epitaxial source/drain region 292.

The vertical opening 232 extends into the epitaxial source/drain region 292 such that the bottom surface 232 b of the vertical opening 232 is below a top 292 t of the epitaxial source/drain region 292. A height 232 h indicates a distance from the top 292 t of the epitaxial source/drain region 292 to the bottom surface 232 b of the vertical opening 232. In some embodiments, the height 232 h may be in a range from greater than 0 nm to about 20 nm. In some embodiments, the height 232 h may be in a range from greater than 0% to about 50% of the height 292 h.

In the example showing FIG. 10, the width of the cross section of the source/drain region 292 in the A-A cut (or the cross section parallel to the y-z plane in the x-y-z coordinate shown in Figure i) increases from the top 292 t to the intermediate portion and decreases from the intermediate portion to a bottom 292 b. A center line 292 c indicates the vertical location of the cross-section area with the largest width 292 w. Corresponding to the increases and decreases in the width in the y-z planes, the size and perimeter of the cross section of the source/drain region 292 in x-y planes also increase from the top 292 t to the center line 292 c and decreases from the center line 292 c to the bottom 292 b. According to some embodiments, the bottom surface 232 b is above the center line 292 c.

The vertical opening 232 may be formed by one or more anisotropic etch processes. For example, a first anisotropic etch process may be performed to etch through the second ILD 230, the first ILD 297, and the CESL 296, and a second anisotropic etch process may be performed to form a recess of the height 232 h in the epitaxial source/drain region 292. In some embodiments, the first anisotropic etch process and the second anisotropic etch process may be the same process performed together in a single operation. In other embodiments, the first anisotropic etch process and the second anisotropic process are different processes with different etch chemistry and/or process parameters.

In some embodiments, the first anisotropic etch process may be a dry etching process, such as a Deep Reactive-Ion Etching (DRIE) process, or any suitable anisotropic etch process. In some examples, the first anisotropic etch process may be performed using inductive coupled plasma (ICP) or capacitive coupled plasma (CCP) containing oxygen, argon, and one or more fluorocarbon-based gas such as hexafluorobutadiene (C₄F₆), octafluorocyclobutane (C₄F₈), or carbon tetrafluoride (CF₄).

In some embodiments, the second anisotropic etch process may be a dry etch process performed using an inductive coupled plasma (ICP) or capacitive coupled plasma (CCP) of an etchant. In some embodiments, the etchant may contain one or more fluorocarbon-based gas such as carbon tetrafluoride (CF₄), trifluoromethane (CHF₃); one or more fluorine-based gas, such as nitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆), hexafluorobutadiene (C₄F₆), and octafluorocyclobutane (C₄F₈); a chlorine-based gas, such as chlorine gas (Cl₂); and/or a bromine-based gas such as hydrogen bromide (HBr).

In some examples, the vertical opening 232 is formed by continuously performing a dry etch process using an inductively coupled plasma. The RF power of the plasma generator can be in a range from about 100 W to about 2000 W to induce and maintain a plasma of the etchant in a process chamber. The process chamber may have a chamber pressure in a range from about 3 mTorr to about 20 mTorr. A bias voltage in a range from about 20 volt to about 1500 volt can be applied to the plasma to achieve anistropic etching. In some embodiments, the anisotropic etch process may be performed for a duration in a range from about 155 seconds to about 1800 seconds. For example, the etch process may be performed for a duration in a range between about 200 seconds to about 1200 seconds, for example about 150 seconds, to form an opening through the second ILD 230, the first ILD 297, and the CESL 296. After the CESL 296 is removed, an additional etching may be performed for a duration in a range from about 5 seconds to about 1050 seconds to recess the vertical opening 232 into the epitaxial source/drain regions 292 for the height 232 h.

After formation of the vertical openings 232, the bottom portion of the vertical openings 232 is expanded by an etching process, such as an isotropic etch process. As shown in FIGS. 6A and 6B, expanded openings 234 are formed from the vertical openings 232 to allow making electrical contact to the source/drain regions 292 for the transistors.

In some embodiments, the expanded openings 234 are formed by an isotropic etch process that has a high etching selectivity between the epitaxial source/drain regions 292 to the second ILD 230, first ILD 297, and CESL 296. As a result, the expanded openings 234 and the vertical openings 232 are substantially the same in dimension in the second ILD 230, the first ILD 297, and CESL 296 while the expanded openings 234 are deeper and have a larger width in the epitaxial source/drain regions 292 than the vertical openings 232.

FIG. 11 is an enlarged view of region 302 in FIG. 6A showing details of a bottom portion of the expanded opening 234. The bottom portion of the vertical opening 232 is shown in dashed line. The expanded opening 234 includes a neck portion 234 n formed through the second ILD 23 o, first ILD 297, and CESL 296 and an end portion 234 e formed in the epitaxial source/drain region 292. The end portion 234 e may have a partial-elliptical cross-section area along the cross-section A-A (refer to FIG. 1). The end portion 234 e may have a partial spheroid shape with a partial spheroidal bottom surface 234 cs formed in the epitaxial source/drain regions 292. As discussed in FIG. 10, the size and perimeter of the cross section of the source/drain region 292 in x-y planes also increase from the top 292 t to the center line 292 c and decreases from the center line 292 c to the bottom 292 b. Isotropically etching the source/drain region 292 after formation of the vertical opening 232 allows the size of the bottom surface 234 cs to expand, and thus, allows the contact area between the source/drain region 292 and the metal fill to be filled in the opening 234 to increase.

A height 234 h indicates a distance from the top 292 t of the epitaxial source/drain region 292 to a bottom point 234 b of the end portion 234 e expanded opening 234. In some embodiments, the height 234 h may be in a range from about 5 nm to about 20 nm. In some embodiments, the height 234 h may be in a range from about 10% to about 5o% of the height 292 h of the epitaxial source/drain region 292. In some embodiments, the bottom surface 234 cs is above the position of the center line 292 c of the epitaxial source/drain region 292 in the cross-section A-A.

In some embodiments, a width 234 w of the end portion 234 e may be in a range from about 14 nm to about 40 nm. In some embodiments, the width 234 w may be in a range from about 30% to about 80% of the width 292 w of the epitaxial source/drain region 292.

The end portion 234 e is formed by laterally expanding the vertical openings 232 in the epitaxial source/drain region 292 by a lateral expansion amount dw and by vertically expanding the vertical opening 232 in the epitaxial source/drain region 292 by a vertical expansion amount dh. In some embodiments, the vertical expansion amount dh can be in a range from about 5 nm to about 20 nm. For example, for an n-type FinFET device, the vertical expansion amount dh can be in a range from about 5 nm to about 20 nm. For a p-type FinFET device, the vertical expansion amount dh can be in a range from about 5 nm to about 10 nm. In some embodiments, the lateral expansion amount dw can be in a range from about 2 nm to about 10 nm. Hence, the width 234 w of the end portion 234 e can be in a range from about 4 nm to about 20 nm greater than the width 232 w of the openings 234. In some examples, the width 234 w of the end portion 234 e is equal to or greater than about 5% more than the width 232 w of the openings 234, such as in a range from about 5% to about 50% more than the width 232 w of the openings 234. The increased width 234 w can increase surface area to which a conductive feature can contact the source/drain regions 292. More specifically, for example, for an n-type FinFET device, the lateral expansion amount dw can be in a range from about 2 nm to about 10 nm. For a p-type FinFET device, the lateral expansion amount dw can be in a range from about 2 nm to about 5 nm. In some embodiments, the source/drain regions 292 in n-type devices and in p-type devices can have different dimensions (e.g., widths and/or heights) due to different materials that are epitaxially grown as the source/drain regions 292. For example, heights and widths of source/drain regions 292 of p-type devices can be smaller than heights and widths of source/drain regions 292 of n-type devices, and hence, vertical expansion amounts dh and lateral expansion amount dw in source/drain regions 292 of p-type devices can be less than vertical expansion amounts dh and lateral expansion amount dw in source/drain regions 292 of n-type devices.

In some embodiments, the lateral expansion amount dw and the vertical expansion amount dh may be achieved by an isotropic etch process, such as an isotropic dry etch process or an isotropic wet etch process.

In some embodiments, the lateral expansion amount dw and the vertical expansion amount dh may be achieved by a dry etch process using an inductively coupled plasma. The etchant may include one of fluorocarbon-based gas such as carbon tetrafluoride (CF₄), trifluoromethane (CHF₃); one or more fluorine-based gas, such as nitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆); a chlorine-based gas, such as chlorine gas (Cl₂); and/or a bromine-based gas, such as hydrogen bromide (HBr). The RF power of the plasma generator can be in a range from about 100 W to about 2000 W to induce and maintain a plasma of the etchant in a process chamber. The process chamber may have a chamber pressure in a range from about 30 mTorr to about 800 mTorr. A bias voltage in a range from about o volt to about 200 volt can be applied to the plasma. In some embodiments, the dry etch process may be performed for a duration in a range from about 5 seconds to about 100 seconds.

In some embodiments, the isotropic dry etch process may be performed in the same dry etch chamber where the anisotropic dry etch processes are performed. Upon formation of the vertical openings 232 shown in FIGS. 5A and 5B, processing parameters of the dry etch chamber are adjusted to perform an isotropic dry etch to form the end portion 234 e of the expanded opening 234.

In some embodiments, the lateral expansion amount dw and the vertical expansion amount dh may be achieved by a wet etch process, for example a wet etching process using ammonium hydroxide-peroxide water mixture (APM), such as a wet etching solution having ammonium hydroxide (NH₄OH), hydrogen peroxide (H₂O₂), and deionized water (H₂O) in a ratio between about 1:1:8000 to about 1:1:100 (NH₄OH:H₂O₂:H₂O). In some embodiments, the wet etching process may be performed in an APM mixture for a duration in a range from about 5 seconds to about 300 seconds.

After the formation of the expanded opening 234, a pre-silicide clean process may be performed to remove native oxide (e.g., SiO₂) from the surfaces of the exposed source/drain regions 292 that may be formed due to exposure to various etchants during formation of the expanded openings 234. Example pre-silicide clean processes may include a wet cleaning using a dilute HF aqueous solution or a dry cleaning using a plasma (e.g., a NF₃/NH₃ plasma), or a combination of both. The chemistries used during the pre-silicide clean may remove native oxide as well as an upper portion of the source/drain regions 292.

A conformal metal layer (not shown) is formed on the surfaces of the exposed source/drain regions 292 (e.g., a partial spheroidal bottom surface 234 cs) and over the surfaces of the second ILD 230, the first ILD 297, and the CESL 296. In some embodiments, the conformal metal layer may include a single layer of titanium, tantalum, or the like. In other embodiments, the conformal metal layer may be a multi-layer stack (e.g., a bi-layer), for example a first layer including titanium, tantalum, or the like, and a second layer including titanium nitride, titanium oxide, tantalum nitride, tantalum oxide, or the like. The metal layer may be deposited by atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), or any suitable deposition technique.

After formation of the metal layer, a silicide layer 214 is formed on the source/drain regions 292 by reacting an upper portion of the source/drain regions 292 with the metal layer, as shown in FIGS. 7A and 7B. The substrate 270 is heated, for example, by performing an anneal process, to cause the silicide reaction to occur wherever the metal layer is in contact with the source/drain regions 292. The anneal process can be, for example, a rapid thermal anneal (RTA) performed at a temperature in a range from about 400° C. to about 650° C., such as about 500° C., for a duration in a range from about 10 seconds to about 60 seconds. The un-reacted metal layer may be removed by a selective etch process that attacks un-reacted metal layer but does not attack the silicide layer 214, or may remain as an adhesion and/or barrier layer, for example.

As shown in FIGS. 7A and 7B, a barrier layer 219 is conformally deposited in the expanded openings 234 on the silicide layer 214 and over the second ILD 230, the first ILD 297, and the CESL 296. The barrier layer 219 may have a thickness of about 2 nm or less, such about 1.8 nm or less, for example about 1.6 nm. The barrier layer 219 may be or include titanium nitride, titanium oxide, tantalum nitride, tantalum oxide, any suitable transition metal nitrides or oxides, the like, or any combination thereof, and may be deposited by ALD, CVD, PECVD, HDP-CVD, low-pressure CVD (LPCVD), or physical vapor deposition (PVD), or any suitable deposition technique. In some examples, the barrier layer is TiN deposited by ALD.

A conductive material 236 (e.g., contact metal) can be deposited on the barrier layer 219 and fill the expanded openings 234. The conductive material 236 may be or include cobalt, tungsten, copper, ruthenium, aluminum, gold, silver, alloys thereof, the like, or a combination thereof, and may be deposited by CVD, ALD, PVD, or any suitable deposition technique. After the conductive material 236 is deposited, excess conductive material 236 and barrier layer 219 may be removed by using a planarization process, such as a CMP, for example. The planarization process may remove excess conductive material 236 and barrier layer 219 from above a top surface of the second ILD 230, as shown in FIGS. 7A and 7B.

FIG. 12 is an enlarged view of region 303 in FIG. 7A showing the details of the conductive material and the silicide layer 214. The silicide layer 214 corresponds to the partial spheroidal bottom surface 234 cs of the end portion 234 e in the expanded opening 234, and therefore, has an increased surface area. The increased surface area of the silicide layer 214 can reduce the resistance between the epitaxial source/drain region 292 and the contact formed in the expanded opening 234.

The conductive material 236 includes a neck portion 236 n formed through the second ILD 230, first ILD 297, and CESL 296 and an end portion 236 e formed on the epitaxial source/drain region 292. The neck portion 236 n may be a trench or a cylinder having substantially vertical walls. The neck portion 236 n may have a width in a range from about 10 nm to about 50 nm. The end portion 236 e may have a partial-elliptical cross section area along the cross-section A-A (refer to FIG. 1). The end portion 236 e may have a partial spheroid shape with a partial spheroidal bottom surface facing the silicide layer 214.

A height 236 h of the end portion 236 e may be in a range from about 5 nm to about 25 nm. A width 236 w of the end portion 234 e may be in a range from about 15 nm to about 50 nm. In some embodiments, the width 236 w of the end portion 236 e may be greater than the width of the neck portion 263 n by an amount in a range from about 2 nm to about 20 nm. The depth and width of the end portion 236 e can enable increased contact surface of the end portion 236 e to the epitaxial source/drain region 292, and thus, can enable reducing resistance therebetween.

The expanded openings may have various shapes depending on, for example, (i) the dimensions of the epitaxial source/drain regions 292, (ii) the dimensions of the vertical openings 232, (iii) whether a passivation process is performed prior to the isotropic etch, and/or (iv) the isotropic etch chemistry, each of which may affect the lateral expansion amount dw and the vertical expansion amount dh. For example, the end portion 234 e may be a partial spheroid, such as a partial-sphere, partial-ellipsoid, or the like.

FIGS. 8A and 8B illustrate expanded openings 244 having a different shape than the expanding openings 234. Similar to the expanded openings 234 of FIGS. 6A and 6B, the expanded openings 244 are formed by expanding the vertical openings 232 of FIGS. 5A and 5B by an isotropic etch process as described with FIG. 6A.

FIG. 13 is an enlarged view of region 304 in FIG. 8A showing details of the expanded opening 244. The bottom portion of the vertical opening 232 is shown in dashed line. The expanded opening 244 includes a neck portion 244 n formed through the second ILD 230, the first ILD 297, the CESL 296, and the upper portion of the epitaxial source/drain region 292, and an end portion 244 e formed in the epitaxial source/drain region 292 below the top 292 t of the epitaxial source/drain region 292. The neck portion 244 n may be a trench or a cylinder having substantially vertical walls. The neck portion 244 n may have a width that is substantially similar to the width 232 w of the vertical openings 232. A bottom 244 nb of the neck portion 244 n is below the top 292 t of the epitaxial source/drain region 292.

The end portion 244 e may have an elliptical cross section area along the cross-section A-A (refer to FIG. 1). The end portion 244 e may have an ellipsoidal shape with an ellipsoidal surface 244 cs formed in the epitaxial source/drain region 292.

A height 244 h indicates a distance from the top 292 t of the epitaxial source/drain region 292 to a bottom point 244 b of expanded opening 244. In some embodiments, the height 244 h may be in a range from about 2 nm to about 20 nm. In some embodiments, the height 244 h may be in a range from about 5% to about 50% of the height 292 h of the epitaxial source/drain region 292. A height 244 eh indicates a height of the end portion 244 e, e.g., a distance from the bottom 244 nb of the neck portion 244 n to the bottom point 244 b of expanded opening 244. The height 244 eh is smaller the than height 244 h.

In some embodiments, a width 244 w of the end portion 244 e may be in a range from about 14 nm to about 40 nm. In some embodiments, the width 244 w may be in a range from about 10% to about 50% of the width 292 w of the epitaxial source/drain region 292.

The end portion 244 e is formed by laterally expanding the vertical openings 232 in the epitaxial source/drain region 292 by a lateral expansion amount dw2 and by vertically expanding the vertical opening 232 in the epitaxial source/drain region 292 by a vertical expansion amount dh2. In some embodiments, the vertical expansion amount dh2 can be in a range from about 2 nm to about 20 nm. For example, for an n-type FinFET device, the vertical expansion amount dh2 can be in a range from about 2 nm to about 20 nm. For a p-type FinFET device, the vertical expansion amount dh2 can be in a range from about 2 nm to about 10 nm. In some embodiments, the lateral expansion amount dw2 can be in a range from about 2 nm to about 10 nm. Hence, the width 244 w of the end portion 244 e can be in a range from about 4 nm to about 20 nm greater than the width 232 w of the openings 244. In some examples, the width 244 w of the end portion 244 e is equal to or greater than about 5% more than the width 232 w of the openings 244, such as in a range from about 5% to about 50% more than the width 232 w of the openings 244. The increased width 244 w can increase surface area to which a conductive feature can contact the source/drain regions 292. More specifically, fFor example, for an n-type FinFET device, the lateral expansion amount dw2 can be in a range from about 2 nm to about 10 nm. For a p-type FinFET device, the lateral expansion amount dw2 can be in a range from about 2 nm to about 5 nm. In some embodiments, the source/drain regions 292 in n-type devices and in p-type devices can have different dimensions (e.g., widths and/or heights) due to different materials that are epitaxially grown as the source/drain regions 292. For example, heights and widths of source/drain regions 292 of p-type devices can be smaller than heights and widths of source/drain regions 292 of n-type devices, and hence, vertical expansion amounts dh2 and lateral expansion amount dw2 in source/drain regions 292 of p-type devices can be less than vertical expansion amounts dh2 and lateral expansion amount dw2 in source/drain regions 292 of n-type devices.

In some embodiments, one or more of the anisotropic etch processes described above with respect to FIGS. 5A and 5B can passivate surfaces of the vertical openings 232. In some examples, the passivated surfaces have C_(x)H_(y)F_(z) formed thereon as a result of the anisotropic etch process(es). After the anisotropic etch processes of FIGS. 5A and 5B, an additional anisotropic etch process, such as using a plasma formed from oxygen (O₂), hydrogen (H₂), or the like, can break through or remove the passivated surface on the bottom of each vertical opening 232. Subsequently, the isotropic etch can selectively etch through the bottom surface of the vertical opening 232 (e.g., etch the respective epitaxial source/drain region 292) without etching the passivated sidewalls of the vertical opening 232 (e.g., including sidewalls of the vertical opening 232 in the epitaxial source/drain region 292). Hence, the profile in FIGS. 8A, 8B, and 13 can be achieved.

In FIGS. 9A and 9B, silicide layers 248 are formed on the surfaces of the epitaxial source/drain regions 292 in the expanded openings 244; a barrier layer 219 is formed conformally in the expanded openings 244; and a conductive material 246 is formed over the barrier layer 219 and filling the expanded openings 244, like described with respect to FIGS. 7A and 7B.

FIG. 14 is an enlarged view of region 305 in FIG. 9A showing the details of the conductive material 246 and the silicide layer 248. The silicide layer 248 corresponds to the ellipsoidal surface 244 cs of the end portion 244 e in the expanded opening 244 and sidewalls of the epitaxial source/drain region 292 formed by the neck portion 244 n of the expanded opening 244, and therefore, can have an increased surface area. The increased surface area of the silicide layer 248 can reduce the resistance between the epitaxial source/drain region 292 and the contact formed in the expanded opening 244.

The conductive material 246 includes a neck portion 246 n formed through the second ILD 230, the first ILD 297, the CESL 296, and a portion of the epitaxial source/drain region 292, and an end portion 246 e formed in the epitaxial source/drain region 292. The neck portion 246 n may be a trench or a cylinder having substantially vertical walls. The neck portion 246 n may have a width in a range from about 10 nm to about 50 nm. The end portion 246 e may have an elliptical cross section area along the cross-section A-A (refer to FIG. 1). The end portion 246 e may have an ellipsoidal shape with an ellipsoidal surface facing the silicide layer 248.

A height 246 eh of the end portion 246 e may be in a range from about 2 nm to about 20 nm. A width 246 w of the end portion 246 e may be in a range from about 10 nm to about 50 nm. In some embodiments, the width 246 w of the end portion 246 e may be greater than the width of the neck portion 246 n by an amount in a range from about 2 nm to about 20 nm. The depth and width of the end portion 246 e can enable increased contact surface of the end portion 246 e to the epitaxial source/drain region 292, and thus, can reduce resistance therebetween.

The combination of anisotropic etch and isotropic etch described here can expose an increased surface area of the source/drain region, and thus a contact area between the source/drain region and the contact feature can be increased. The increased contact area can reduce resistance between the contact feature and the source/drain region, and therefore, performance of the devices can be improved.

In an embodiment, a structure is provided. The structure includes an active area including a source/drain region, a dielectric layer over the active area, and a conductive feature through the dielectric layer to and contacting the source/drain region. The conductive feature includes a neck portion through the dielectric layer, and an end portion extending in the source/drain region. A width of the end portion is greater than a width of the neck portion.

In another embodiment, a method is provided. The method includes forming a vertical opening through a dielectric layer over a source/drain region of an active area to expose a portion of an upper surface of the source/drain region, and expanding the vertical opening to form an expanded opening in the dielectric layer and in the source/drain region. The expanded opening includes a neck portion through the dielectric layer and an end portion in the source/drain region. The neck portion has a first width, and the end portion has a second width that is greater than the first width.

In yet another embodiment, a method for semiconductor processing is provided. The method includes forming a source/drain region in an active area on a substrate, forming a dielectric layer over the active area and the source/drain region, anisotropically etching through the dielectric layer to form an opening, the opening exposing at least a portion of an upper surface of the source/drain region, isotropically etching, through the opening, the source/drain region to laterally and vertically expand an end portion of the opening, and forming a conductive feature in the opening.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor structure comprising: an active area including a source/drain region; a dielectric layer over the active area; and a conductive feature through the dielectric layer to and contacting the source/drain region, wherein the conductive feature includes: a neck portion through the dielectric layer; and an end portion extending in the source/drain region, wherein a width of the end portion is greater than a width of the neck portion, wherein the conductive feature extends along a bottom surface of the dielectric layer.
 2. The structure of claim 1, wherein the width of the end portion is in a range from 5% to 50% more than the width of the neck portion.
 3. The structure of claim 1, wherein the end portion extends in the source/drain region a depth in a range from 5% to 50% of a height of the source/drain region.
 4. The structure of claim 3, wherein the depth is in a range from 10% to 50% of the height of the source/drain region.
 5. The structure of claim 1, wherein the end portion has a partial-circular cross-section profile.
 6. The structure of claim 1, wherein the end portion has an elliptical cross-section profile.
 7. The structure of claim 1 further comprising a silicide layer disposed between the conductive feature and the source/drain region, wherein the silicide layer has a partial spheroid profile or an ellipsoid profile.
 8. The structure of claim 1, wherein the dielectric layer comprises an etch stop layer over the active area and an interlayer dielectric layer over the etch stop layer.
 9. A semiconductor structure comprising: a source/drain region in an active area on a substrate; one or more dielectric layers over the source/drain region; and a conductive feature extending through the one or more dielectric layers, the conductive feature extending below an uppermost surface of the source/drain region, the conductive feature having a neck portion and an end portion, wherein a widest dimension of the end portion is at the uppermost surface of the source/drain region, wherein the widest dimension of the end portion is greater than a width of the neck portion at the uppermost surface of the source/drain region.
 10. The structure of claim 9, wherein a distance from an uppermost surface of the source/drain region to a lowermost surface of the conductive feature is from about 5 nm to about 20 nm.
 11. The structure of claim 10, wherein the source/drain region is an epitaxial layer, the epitaxial layer being a different semiconductor material than the substrate, wherein the distance is 10% to about 50% of a thickness of the epitaxial layer.
 12. The structure of claim 9, wherein the widest dimension is in a range from about 14 nm to about 40 nm.
 13. The structure of claim 9, wherein the source/drain region extends over a portion of the conductive feature.
 14. A semiconductor structure comprising: a semiconductor substrate, the semiconductor substrate comprising a first semiconductor material; an epitaxial region on the semiconductor substrate, the epitaxial region comprising a second semiconductor material; one or more a dielectric layers over the epitaxial region; and a conductive feature extending through the one or more dielectric layers, the conductive feature comprising a first portion and a second portion, the first portion being above an interface between the one or more dielectric layers and the epitaxial region, the second portion being below the interface, the first portion having the one or more dielectric layers along opposing sidewalls, the first portion having a first width at the interface between the one or more dielectric layers and the epitaxial region, the second portion having the epitaxial region along opposing sidewalls, a second width of the second portion being a widest width of the second portion, the second width being greater than the first width, the conductive feature extending along a lower surface of the one or more dielectric layers.
 15. The structure of claim 14, wherein the first width is in a range from about 10 nm to about 40 nm.
 16. The structure of claim 14, wherein the second width is 2 nm to 10 nm greater than the first width.
 17. The structure of claim 14 further comprising a silicide region interposed between the conductive feature and the epitaxial region.
 18. The structure of claim 17, wherein the conductive feature comprises a barrier layer and a conductive fill.
 19. The structure of claim 18, wherein a thickness of the barrier layer is 2 nm or less.
 20. The structure of claim 18, wherein the conductive feature extends into the epitaxial region to a depth 10% to about 50% of a thickness of the epitaxial region. 